Hybrid organic-inorganic perovskites, as well as the perovskites in
general, are known for their phase complexity evidenced by the
stabilization of different polymorphs, and thus an understanding of
their regions of stability and transitions can be important for their
photovoltaic and optoelectronic technologies. Here we use a multiscale
approach based on first-principles calculations with van der Waals
corrections and classical force-field molecular dynamics to determine
the finite-temperature properties of the tetragonal and cubic phases of
CH3NH3PbI3. Temperature effects are implicitly included using the quasi-
harmonic approximation that can describe anharmonic behavior due to
thermal expansion through the dependence of the harmonic frequencies on
structural parameters. Our finite-temperature free-energy surfaces
predict the lattice and elastic moduli evolution with temperature, and
show in particular that the calculated lattice parameters of the cubic
and tetragonal phases are to within 1% of experimental values. Further,
our results show that the phonons are the major contributing factor for
stabilizing the cubic phase at high temperatures mainly due to the low-
energy phonon modes that are associated with the inorganic lattice. On
the other hand, the configurational entropy due to CH3NH3+ rotational
degrees of freedom is slightly more favored in the cubic phase and
amounts to less than 0.2% of the T = 0 K free-energy difference between
the two phases. Published by AIP Publishing.